In order to accurately control a rotational operation of an electric rotating machine, rotor position information of the electric rotating machine and information about a current flowing in the electric rotating machine are needed. Here, conventionally, the rotor position information is obtained by additionally attaching a rotational position sensor to the rotary electric machine. However, the additional provision of a rotational position sensor has a significant disadvantage in view of cost reduction, space reduction, improvement of reliability. Therefore, change from rotational position sensor style to sensorless style has been required.
Examples of control methods for changing the electric rotating machine from rotational position sensor style to sensorless style include a method of estimating the position of a rotor of the electric rotating machine mainly from an inductive voltage of the electric rotating machine, and a method of estimating the position of the rotor of the electric rotating machine by using saliency (positional dependence of inductance) of the electric rotating machine.
The inductive voltage which is used in the former method has a characteristic that the magnitude thereof is proportional to the speed of the electric rotating machine. Therefore, at a zero speed or a low speed, the inductive voltage decreases and an S/N ratio is deteriorated. As a result, it becomes difficult to accurately estimate the position of the rotor of the electric rotating machine.
On the other hand, in the latter method which uses saliency, a high-frequency position estimation voltage instruction having a frequency different from a drive frequency of the electric rotating machine is applied to the electric rotating machine, a high-frequency electric rotating machine current flowing in the electric rotating machine along with the application of the position estimation voltage instruction is detected, and the position of the rotor is estimated by using the fact that the magnitude of the electric rotating machine current varies depending on the position of the rotor owing to the saliency.
Thus, the method using saliency has an advantage that the position of the rotor of the electric rotating machine can be estimated without depending on the rotational speed of the electric rotating machine, though the position estimation voltage instruction for estimating the position of the rotor of the electric rotating machine needs to be inputted to the electric rotating machine. Therefore, especially at a zero speed or a low speed, a position sensorless control method using saliency is used.
However, in such a position sensorless control method using saliency, since the high-frequency position estimation voltage instruction needs to be applied to the electric rotating machine, noise occurs along with the application, and the noise gives a sense of discomfort to a person.
As a measure for reducing such a sense of discomfort due to the noise occurring along with the application of the position estimation voltage to the electric rotating machine, conventionally, a method of reducing the magnitude of the noise occurring from the electric rotating machine by reducing the amplitude of the position estimation voltage applied to the electric rotating machine, and a method of improving the sound quality of the noise, have been proposed.
In the former method of reducing the magnitude of the noise occurring from the electric rotating machine by reducing the amplitude of the position estimation voltage instruction applied to the electric rotating machine, since the amplitude of the position estimation voltage instruction is small, it is difficult to accurately estimate the position of the rotor of the electric rotating machine.
On the other hand, in the latter method of improving the sound quality of the noise, for example, as in a conventional technique disclosed in Patent Literature 1, the frequency of the position estimation voltage instruction applied to the electric rotating machine is intentionally varied in a random manner so that a sound having a particular frequency component is not conspicuous, whereby a sense of discomfort that a person feels is reduced, because when a sound having a particular frequency component is conspicuous among sounds that a person can hear, the person feels a sense of discomfort from that sound.    Patent Literature 1: Japanese Laid-Open Patent Publication No. 2004-343833